This invention relates to protector assemblies and methods for protecting motor windings from shorting the protector.
Winding protectors are designed to protect electric motors from an overload condition. A typical winding protector is electrically coupled to a motor""s winding and adapted to break current through the winding when the protector senses excessive temperature or current in the winding. The protector includes a housing having an open end, a cap enclosing the open end, one or two pin-type terminals extending through the cap, and a thermally-responsive member within the housing and electrically coupled to the terminals. The protector is adapted to be positioned in heat-transfer relation to the motor""s winding such that heat generated by the winding is transferred to the thermally-responsive member via the housing. The thermally-responsive member is configured to move to an open-circuit position when the thermally-responsive member is heated to a particular temperature. To facilitate heat transfer from the winding to the thermally-responsive member, the housing is metal, typically steel. Because the housing and pin-type terminals are metal and therefor electrically conductive, a dielectric sleeve is positioned over the protector to prevent the protector from short circuiting the winding.
A problem associated with such a protector is that the connection of the cap to the housing produces a relatively sharp edge capable of cutting through the dielectric sleeve. The pin-type terminals are also capable of cutting through the dielectric sleeve. A risk of short circuiting the winding occurs if any part of the protector cuts through the sleeve.
One known solution for reducing the risk of the protector short circuiting the winding is replacing the sleeve with a plastic protector cover, molded to fully cover the protector. The protector cover is often an undesirable solution because it is substantially thicker than the sleeve and therefor provides more thermal insulation between the protector and winding than does the sleeve. Accordingly, a higher winding temperature is needed before the thermally-responsive member is heated sufficiently to move to its open-current position, necessitating the selection of a different protector with different operating characteristics. Also, the shape and rigidity of the protector cover make placement of the cover in the end-wire envelope difficult. Moreover, the cost of the molded cover is substantially greater than that of the sleeve.
Among the several advantages of the present invention is the provision of an improved dielectric barrier for and method of electrically insulating a thermally responsive protector from a motor winding of an electric motor; the provision of such a barrier and method which prevents the protector from short circuiting the motor winding; the provision of such a barrier and method which does not provide excessive thermal insulation between the protector and motor winding; the provision of such a barrier and method which overcome the disadvantages associated with the prior art.
In general, a method of the present invention is for insulating a thermally responsive protector from a motor winding of an electric motor. The protector comprises a thermally and electrically conductive housing, a thermally responsive member, an end cap, and at least one terminal. The housing has an open rearward end, a closed forward end, and an intermediate portion between the ends. The thermally responsive member is within the housing. The end cap covers the open end of the housing. The terminal extends through the end cap. The terminal is electrically insulated from the housing and has an outer end extending rearwardly out of the end cap. The thermally responsive member is adapted to be electrically coupled to the motor winding via the terminal. The protector is adapted to be positioned in heat-transfer relation with the motor winding in a manner such that the thermally responsive member is responsive to an overload condition of the motor winding. The method comprises providing a skirt having a forward portion and a rearward portion, and positioning the skirt on the protector in a manner such that: (a) the skirt is spaced from the intermediate portion of the housing; (b) the forward portion of the skirt surrounds the end cap; and (c) the rearward portion of the skirt extends rearwardly from the end cap and surrounds the outer end of the terminal. The skirt is of a dielectric material. The method further comprises providing a sleeve of a dielectric material and positioning the sleeve relative to the protector such that the sleeve surrounds the skirt and surrounds the protector in a manner in which the sleeve is in engagement with the intermediate portion of the housing. The positioning of the skirt and of the sleeve is maintained such that the sleeve and skirt provide a dielectric barrier between the protector and the motor winding when the protector is positioned in heat-transfer relation with the motor winding.
Another aspect of the present invention is a protector assembly for protecting an electric motor from an overload condition. The motor has a motor winding. The assembly comprises a thermally responsive protector, a skirt, and a sleeve. The protector comprises a housing which is both thermally and electrically conductive, an end cap, at least one terminal, and a thermally responsive member. The housing has an open rearward end, a closed forward end and an intermediate portion between the ends. The end cap covers the open end of the housing. The terminal extends through the end cap and is electrically insulated from the housing. The terminal has an outer end extending rearwardly out of the end cap. The thermally responsive member is within the housing and adapted to be electrically coupled to the motor winding via the terminal. The protector is adapted to be positioned in heat-transfer relation with the motor winding in a manner such that the thermally responsive member is responsive to an overload condition of the motor winding. The skirt is of a dielectric material and has a forward portion and a rearward portion. The sleeve is of a dielectric material. The skirt is positioned on the protector in a manner such that: (a) the skirt is spaced from the intermediate portion of the housing; (b) the forward portion of the skirt surrounds the end cap; and (c) the rearward portion of the skirt extends rearwardly from the end cap and surrounds the outer end of the terminal. The sleeve surrounds the skirt and surrounds the protector in a manner in which the sleeve is in engagement with the intermediate portion of the housing. The skirt is between the end cap and the sleeve. The skirt and sleeve are adapted to provide a dielectric barrier between the protector and the motor winding when the protector is positioned in heat-transfer relation with the motor winding.
Other objects and features will be in part apparent and in part pointed out hereinafter.